I could be wrong, but I'm assuming you've not seen them for 6 years because they live overseas, and I'm guessing your DS & DiL will be staying with you too?
As others have suggested, I'd have a few little things around the house to keep them occupied when you're in home ... a pack of cards are usually a big hit, and can be enjoyed by all the family, or monopoly/books/comics, etc. Have a few ideas about where you can go with them before they arrive, so that you're not left thinking what to do with them. It's probably a good idea to speak with your DS/DiL before they arrive, to ask what type of things they'd all like to do, then you can make plans in advance.
As far as a gift is concerned, you could get them each a little welcome goodie bag for when they arrive, including stuff like a little lego set/comic/colouring pens/etc., for you grandson. And perhaps some girlie things from Accessorize for your granddaughter, or nail polish/little mini toiletries from The Body Shop, or similar.
Towards the end of their stay, when you've got to know them a little better, you could give them each a budget, and ask them to choose a little gift from you, to remember the time they spent with you.
I hope you all enjoy your special time together!